The present invention generally relates to an apparatus for controlling a plasma and, in particular, relates to such an apparatus in which the intensity of the plasma discharge is used to generate a control signal.
One of the more significant recent advances in the field of atomic spectroscopy is generally referred to as plasma emission spectroscopy. In such a system, the sample is heated by means of a plasma discharge to such a high temperature that atomic emission occurs. During atomic emission some of the electrons of an atom are, by the thermal energy imparted thereto, raised to a higher energy level. Upon decay, i.e., an electron returning to a lower energy level, a photon of light is emitted. This emitted light propagates at a specific wavelength which is characteristic of the particular element. Consequently, by determining the intensity of the light emitted at a characteristic wavelength the concentration of a particular element can be determined. Such an analytical technique is probably most advantageous for refractory elements that are relatively insensitive to other atomization techniques. This advantage derives from the high temperatures inherently associated with a plasma torch, i.e., on the order of about 5000.degree. C.
The most conventional form of plasma torch presently in use is generally referred to as an inductively coupled plasma (hereinafter ICP). The gases necessary to sustain an ICP discharge are commonly introduced into a torch constructed of quartz. The high temperature plasma discharge is partially contained by a quartz tube. In such a torch, a quartz tube surrounds the torch to shape the plasma, which torch is ignited and maintained by means of a strong radio frequency (RF) field. The RF field is created by an RF load coil through which the gases are fed.
Because of the ability of ICP discharges to reach very high temperatures, which can easily exceed the melting temperature of quartz, the torch must be carefully monitored to prevent damage thereto. Excessive heating of the outer quartz tube generally occurs either during start-up or when insufficient gas flow is provided.
Regardless of the cause, it is difficult to measure the tube wall temperature for at least three reasons. First, it is difficult to provide any direct contact monitor to the quartz because of the high temperature thereof. Second, any form of electronic monitoring within the torch chamber is difficult due to the strong electromagnetic fields near the RF coil. Third, during the start-up of the torch, which occurs within the quartz tube, excessive temperatures can be reached quickly and without detection by conventional techniques. Additionally, since the ability to remove and/or replace the torch is a desirable convenience it is important to avoid complicating that procedure by including unnecessary devices within the torch chamber.
The above-recited difficulties are somewhat obviated by an apparatus for monitoring a plasma torch which is described in U.S. patent application Ser. No. 526,758 filed on Aug. 26, 1983 and assigned to the assignee hereof. Therein a photometer is externally affixed to the housing of a plasma torch chamber and directed so as to view the quartz tube of the plasma torch. By use of a pyrolytic sensor the temperature of the torch is monitored. This apparatus is, however, not immune to electrical noise, thermal drift and shortened component life-times due to the proximity of the apparatus to the torch.